Fuel vapors, which escape from a tank, must not be let into the environment. For that reason fuel vapors from the tank are let into an adsorbing vessel that preferably contains an activated carbon filter, the activated carbon holds the fuel vapors back, so that air can only escape into the environment over a ventilation pipe. In order to regenerate the activated carbon in the adsorbing vessel, a connecting line is provided between the adsorbing vessel and a inlet manifold of the combustion engine. For the regeneration a controllable tank ventilation valve is opened and the fuel mass that has been stored in the activated carbon is sucked into the inlet manifold and brought to the combustion by the combustion engine during the operation of the combustion engine. The functionality of the tank ventilation system is controlled. When a significant mixture deviation occurs at the opening of the tank ventilation valve, this can indicate that fuel vapor is sucked off the activated carbon and the activated carbon filter is flushed out.
When using not loaded activated carbon filters by contrast only air is sucked in that case an elaborated ‘air-checking’ is undertaken. For this purpose the tank ventilation valve is actively controlled in idle mode only for diagnostic reasons and the reactions of the air system are detected and ‘observed’. But this diagnosis has to be rejected often, because the diagnosis result may possibly be useless by the cut-off or add-on consumer loads during this diagnosis.
A procedure for a tank ventilation in a combustion engine arises from DE 103 35 902 A1, in which a fuel mass flow through a tank ventilation valve or a characteristic curve is obtained, compared to a fuel mass flow that has been determined with the aid of the measuring data of a pressure sensor, and the fuel mass flow that is obtained from the characteristic curve is corrected depending on the fuel mass flow that has been determined with the aid of the pressure sensor data. This allows the registration of tolerances of the tank ventilation valve. By doing so it can be excluded, that the fuel amount that is de- or increased by the tolerance, which is brought to the combustion engine, is misinterpreted as a lower or higher loading. The problem of this procedure is that the detection of the fuel mass flow is carried out by a pressure sensor. Furthermore a diagnosis is only possible with an active controlling of the tank ventilation valve.
The invention is therefore based on the task to enable a diagnosis of the tank ventilation valve with its active controlling, which is furthermore insensitive towards consumer load add-ons and cut-offs.